The major objective is to study at the genetic, chemical and molecular level the antigen profile of human lymphoid cell membrane to obtain a better understanding of its role in defense and recognition of disease. Progress towards these objectives included an immunochemical study of the antigen mosaic on cultured human lymphoid cells in terms of expression as well as spatial and structural relationships between cell surface markers. Xenoantisera to HLA and DR antigens were produced which not only proved to be specific by serological and immunochemical analyses but which could be utilized effectively as molecular probes to isolate and purify these antigens to determine their partial amino acid sequences. The amino acid sequence for HLA-A9 antigens is identical within the first 20 terminal residues to that reported for HLA antigens with other allotypic specificities. The amino acid sequence of the alpha chains of human DR and murine Ia antigens is identical within the first 30 N-terminal residues. Interestingly enough our sequence for beta-chains of the DR molecule shows considerable sequence homology with murine Ia antigens determined by the I-E/I-C subregions of the murine MHC.